1. Field of the Invention
The present invention relates to polyester compositions and a process for preparing polyester films by utilizing such compositions.
More particularly, this invention relates to polyester compositions which are excellent in dispersibility in the molten state, pass well through filters in the extrusion steps, and can be made into films having few coarse surface protrusions, excellent sliding properties (slipperiness), and high abrasion resistance.
2. Prior Art
Polyester films, especially biaxially oriented polyester films, including polyethylene terephthalate films, have widely been used as base films for magnetic tapes, capacitors, packaging materials, plate materials in photomechanical processes, electrically insulating materials, etc., owing to their excellent electrical, mechanical and thermal properties as well as good workability and high chemical resistance.
The specific characteristics required for such polyester films may differ depending on their use, but mention may particularly be made of excellent sliding properties and few coarse protrusions on their surface, which are generally essential requirements common to conventionally available films. In particular, it is very important for films to have good sliding properties since such properties may influence winding workability in film production steps, easy handling of films in post-treatment steps such as coating, vacuum deposition, slitting, winding-up, etc., and even quality of final products, for example, running properties of magnetic tapes produced therefrom or the resistance of the tapes against guide pins and other elements.
Generally, fine unevenness may be given to films on their surface to improve their sliding properties. In order to provide such fine unevenness on the film surface, a method has been well known in which residual metal compounds used as catalysts in the polymerization processes of polyesters may be deposited in the form of fine particles in the reaction system.
Also well known is a method in which fine particles inert to polyesters are added in the process of producing polyesters or in the extrusion steps thereof so as to form protrusions on the surface of resulting films, and thereby to give sliding properties thereto: this method will hereinafter be called "the particle addition method". Such particles used in the particle addition method may include natural or synthetic calcium carbonate, natural clays and minerals, which have been adjusted to proper particle sizes by crushing, classifying and/or other means.
Recently, requirements for film quality have become sterner, and especially reduction of long-lasting undulation of films, namely improvement of the precision in thickness, has been considered to be essential to the film quality.
For this end, there has been known the so-called electrostatic cooling method in which a sheet melt-extruded through a nozzle is closely adhered to a cooled surface. However, when the speed of the rotating cooling drum is increased so as to improve productivity in this electrostatic cooling method, the adhesive force of the extruded sheet to the cooling drum will be reduced, thereby resulting in the formation of the so-called pinning bubble. Such pinning bubble in the product films must be avoided to satisfy the quality requirements.
Generally, the higher the specific resistance of the melt of starting polyester materials, the greater is the tendency of said pinning bubble to generate. More specifically, when the specific resistance as measured by a method mentioned later has a value equal to or greater than 5.times.10.sup.8 ohm.cm, said pinning bubble tends to be formed. Thus, such higher specific resistance will make it difficult to efficiently obtain films of high quality.
As one of the methods for preventing the formation of such pinning bubble, it has been proposed to add a metal compound during the polyester preparation process to thereby reduce the specific resistance of the polyester melt, as disclosed, for instance, in Japanese Patent Application Laying-open No. 70269/76. However, this method is unsatisfactory and further reduction of the specific resistance of polyester melt to a desired degree has still been demanded.
In the particle addition method used to improve the slipperiness of polyester films, the particles are usually dispersed in an appropriate medium such as ethylene glycol, water, an alcohol, or the like, and then added to the polycondensation reaction system for producing polyesters. It is imperative that the particles are well dispersible in the medium such as ethylene glycol and any substantial gross agglomeration of the particles does not take place in the polycondensation reaction system: that is, the particles should be uniformly dispersed in the polyester.
Although coarse particles which cannot be uniformly dispersed in the slurry medium such as ethylene glycol or in the polycondensed polymer may preliminarily be eliminated by classification or any other method to obtain those having a relatively sharp particle size distribution, the possible agglomeration of the added particles may cause serious problems: the surface roughness of the resulting films may vary; or, in certain severe cases, the filter used in the extrusion step may be plugged. Further, coarse protrusions may be formed on the film surface.
If such films are made into magnetic tapes, a decrease of output or an increase of drop-out may be caused. Similarly, when such films are used in capacitors, various troubles, e.g., a decrease of the voltage which the capacitor can withstand, etc. may be produced.
As previously stated, the particles of natural calcium carbonate used to add to polyesters have been obtained through repeated crushing and classification steps. However, there have been inevitable limitations on the sharpness of the particle size distribution and on the removal of coarse particles. Thus, conventional particles of natural calcium carbonate do not have a satisfactory quality for particles to be added to polyester films.
When such conventional particles are added to polyesters in an amount necessary for improving the sliding properties and abrasion resistance of the resulting films, the number of coarse protrusions in the films may be increased, and the back pressure of the filter in the extrusion step may be greatly increased resulting in an increase of the number of exchange of such filters and in a reduction of productivity.
In order to avoid such problems, it is necessary to remove coarse particles by many times repeating crushing and classification of calcium carbonate to obtain particles having a sharp distribution of particle size. This will, however, increase the number of treating steps and hence cause an economical disadvantage. Further, the resulting fine particles of calcium carbonate are not uniformly dispersed in media such as ethylene glycol or in polyesters, and agglomeration thereof may occur in ethylene glycol or the polyester resulting in the formation of coarse particles. Thus, such particles derived from natural calcium carbonate are far from practical.
On the other hand, synthetic calcium carbonate may generally be prepared by well known methods, such as the carbonatation of milk of lime, Ca(OH).sub.2, the addition of an aqueous solution of sodium carbonate or ammonium carbonate to an aqueous solution of calcium chloride, and the so-called calcium chloride method.
However, the primary particles of such synthetic calcium carbonate usually exhibit a very strong cohesive force and hence many primary particles agglomerate together with one another to form larger secondary particles. Such secondary particles are very difficult to completely break up into the smaller primary particles in the good particulate state even if the synthetic calcium carbonate is vigorously stirred for a long period of time. For example, when grinding or crushing is carried out by a ball mill, a powerful sand grinder, or the like, the agglomerate, secondary particles can substantially be broken up into the smaller primary particles while at the same time the primary particles may also be broken into much smaller particles. The resulting fine particles will have an unstable surface state and contain both particles having smaller particle sizes than those of the desired primary particles and incompletely broken secondary particles: hence, the particles will have a wide distribution of particle size. Further, they will tend to agglomerate again in the polyester due to their unstableness.
Therefore, the use of such synthetic calcium carbonate is limited to a very narrow range of applications; it cannot have been applied to films requiring especially highly precise surface characteristics, for example, magnetic tapes or capacitors.
In the electrostatic cooling method employed to improve the precision of film thickness, a problem may be caused when the specific resistance of the starting polyester melt is reduced in order to prevent the formation of pinning bubble: thus, as the specific resistance of the melt is reduced, the thermal stability of the polyester will generally become poorer and the degree of polymerization may be decreased and/or the formation of degraded products may be enhanced resulting in the loss of film stability in the continuous preparation thereof by extrusion.
Thus, there has been a primary need to provide a method of reducing the specific resistance of the polyester melt as greatly as possible while retaining the thermal stability of the polyester.